Microwave heating apparatus



l- 0 H. A. PUSCHNERQ 3,535,433

MICROWAVE HEATING APPARATUS Original Filed Feb{ l2, 1968 3 Sheets-Sheet l v l'n i/ehfor: Herber-l; Auyusi Pils chner Oct. 20, 197D H. A. PUscHNER MICROWAVE HEATING APPARATUS Original Filed Feb. 12, 1968 5 Sheets-Sheet 2 Inventor: Herber Augus PL'isc/mer BY 1 H. A. PUSCHNER MICROWAVE HEATING APPARATUS Original Filed Feb. 12, 1968 5 Sheets-Sheet 5 v o a s 4 I I 44: I. J $1 ram FORM mougrlolv' EGULA TOR LOAD v WA VEGU/DE .RECTlF/ER NA GNLE TRON I HIGH TRANS III!) Il/ LTA E Inventor usi: Puschner #nitedStates Patent Oi 3,535,483 Patented Oct. 20, 1970 ,591,83 Int. Cl. H05b 9/06, 5/00 US. Cl. 219-1055 5 Claims ABSTRACT OF THE DISCLOSURE Microwave heating apparatus for heating on metallic materials. The apparatus includes a magnetron for generating the microwave heating energy and a source of power for supplying the magnetron with electricity at a constant voltage.

This application is a continuation of application Ser. No. 704,859, now abandoned.

BACKGROUND OF THE INVENTION The present invention relates to microwave apparatus for heating nonmetallic materials, e.g. foodstuffs in the preparation of packaged foods. The apparatus includes one or more magnetrons for the generation of microwaves or, more particularly, magnetrons for generating continuous waves.

Prior art apparatus for heating a material with microwaves has been beset with the problem of maintaining the microwave output power at a constant level. This requirement for a constant output power is critical if the material to be heated is to be heated uniformly.

Changes in the output power of a continuous-wave magnetron, a device which may be viewed as a current source having a low internal impedance are causedaside from variations in the internal impedance of its power supply by line voltage fluctuations and by a mismatching of the high frequency load impedance. If the load impedance is matched (s=voltage standing wave ratio (VSWR) 51.05 variations of :10% in the line voltage will cause the output power, depending on the type of magnetron, to vary up to 140%.

A known technique for reducing these variations in output power is to supply the continuous-wave magnetron with a constant anode current. A number of means are available for controlling the anode current. It is possible to employ a controlled rectifier on the high voltage side, or, on the low voltage side, transductor or thyristor control means.

Though the regulation is used to maintain a constant anode current, the microwave output may still vary as a result of the admissible range of reflection of the high frequency load impedance over a region of about i%, depending on the type of magnetron. These variations, caused by the mismatching of the high frequency load impedance, are too great for many industrial uses.

SUMMARY OF THE INVENTION An object of the present invention, therefore, is to stabilize the power produced by microwave heating apparatus.

This, as well as other objects which will become apparent in the discussion that follows, is achieved, according to the present invention, by making the power supply for the microwave generating magnetron a constant voltage source.

If the constant current regulation described above, which has in the past been considered an absolute necessity to insure protection of the continuous-wave magnetron, in abandoned and the magnetron supplied only through a constant voltage source, the surprising result is that variations in the output power are held within very narrow tolerances and, as a consequence, the prescribed tolerances in the operating parameters of the magnetron will not be exceeded.

Power supplies producing a constant output voltage may be manufactured with less expense than the customary power supplies employing a constant current control. This is especially true for microwave generating apparatus of medium to high output power which, as a rule, employs a number of continuous-wave magnetrons. Because of their simpler construction the constant voltage power supplies are also more reliable an easier to maintain.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic diagram, in elevation, of a continuous-flow heating plant employing a number of microwave generators.

FIG. 2 is the generator diagram (Rieke-diagram) of a continuous-wave magnetron operated with a constant anode current regulator.

FIG. 3 is the generator diagram of a continuous-wave magnetron operated with a constant input voltage regulator, according to the present invention.

FIG. 4 is a schematic diagram of the apparatus, according to the present invention, which generates microwaves for the heating apparatus of FIG..1.

DESCRIPTIONS OF THE PREFERRED EMBODIMENTS Referring now to the drawings, there is shown schematically in FIG. 1 a continuous-flow plant for heating meat 11 with the aid of microwave energy. This type of plant may be used, for example, to heat meat as a part of the process of food packing.

The meat 11 is introduced onto the conveyor belt 13 through the tube 12. The conveyor belt 13, which is driven by the rollers 14 and 15, transports the meat 11 through the channel 16 where it is irradiated by four microwave energy sources 17, 18, 19 and 20, fed by magnetrons. The microwave energy heats the meat 11 gently yet quickly and uniformly. The heated meat is then dropped into the funnel 21 and is conveyed to the next station for processing.

The meat to be heated makes frequent quick changes in its consistency as it passes through the radiation area of each microwave energy source. From the electrical viewpoint, this means that both the magnitude and phase of the load impedance Z of the generator are extremely variable. There are, as a result, strong variations in the amount of power received by the meat.

In the two generator diagrams (Rieke-diagrams) FIGS. 2 and 3, wherein the load impedance is represented in polar coordinates by the phase l/A related to the reference plane of the generator output waveguide, and by s, the voltage standing wave ratio (VSWR), the dashed curve b illustrates one example of the many possibilities for change in the load impedance. In the above phase relation, I is the distance from the first voltage minimum on the waveguide to the reference plane of the generator output waveguide, and 7\ is the wavelength of the waveguide. These generator diagrams define the dependance of the output power of a continuous-wave magnetron from the load impedance, when the magnetron has an output power P =5.1 kw. at matched load impedance Z (s: 1.0) (the central point in the generator diagrams) and is fed by constant current resp. constant voltage. The circles c in the diagrams represent in the maximum value of voltage standing wave ratio s=2.5 which should not be exceeded with respect to the reliability of the magnetron.

The advantage of the apapratus according to the present invention may be seen by comparing two diagrams. The generator diagram of FIG. 2 represents the prior art continuous-wave magnetron system which utilizes constant current regulation. The family of curves a are lines of constant output power. If the load impedance Z varies in a manner shown by the dashed curve b, the power output finds a minimum value at point d (P -1.3 kw.) and a maximum value at point 1 (P +0.5 kw.). The percentage variation thus runs from 25% to +10%. Such a variation is inadmissably high for many heating applications since, for example with the continuous-flow heating plants of FIG. 1, it produces a nonuniform temperature distribution within the meat 11 along the direction of flow.

If a constant voltage regulator is used, on the other hand, in place of the constant current regulator, the power variations will be considerably reduced. This fact results from the different shape of the constant power curves 11 in the generator diagram of FIG. 3. The output power produced by the identical magnetron subjected to the same load impedance variations [1 which were represented in FIG. 2 takes on a minimum value at point g of P 0.3 kw. and a maximum value at point h of P +0.3 kw. The percentage variations have thus been reduced to i6%. As a result, the temperature along the direction of flow of the meat heated will be made considerably more uniform.

An additional important advantage of the apparatus according to the present invention is that the output power of the magnetron at a given standing wave ratio s, proceeding from the matching point (s=l.0) in the direction of greater mismatch, will be practically independent of the phase relationship of the reflection at the load impedance. If one moves about circles of constant standing wave ratio s in the generator diagram of FIG. 3 (e.g. a distance of 1r/ 2 around the circle s: 1.4), the power variations will be considerably less than with the magnetron having constant current regulation diagrammed in FIG. 2. These power variations are lower because the lines a of the apparatus according to the present invention do not run across the circles s constant; rather, they run, for the most part, in an approximate circle.

FIG. 4 illustrates the preferred embodiment of the circuit according to the present invention. An induction voltage regulator 1, an adjustable transformer which is auto matically regulated by a motor when under load, forms the constant voltage source. This type of regulator is described on pages 365 and 366 of volume 14 of the McGraw-Hill Encyclopedia of Science and Technology (New York, 1960). The primary of the induction regulator 1 is connected directly to the line 2. The secondary, which receives a constant three-phase voltage, is connected to a high voltage transformer 3. The transformer 3 feeds a rectifier circuit 4 which, in turn, is connected to the continuous-wave magnetron 5. The magnetron 5 generates microwave energy which is fed via a waveguide 6 to the load 7, such as the load impedance Z.

The term magnetron as used in this application is intended to include all cross-field microwave generating devices known in the art.

It will be understood that the above description of the present invention is susceptible to various modifications, changes and adaptations, and the same are intended to be comprehended within the meaning and range of equivalents of the appended claims.

I claim: I

1. Apparatus for the microwave heating of a foodstuff of the type which presents, with respect to microwaves, an electrical impedance whose magnitude and phase angle vary by a substantial amount as the foodstuff is heated, comprising, in combination: a microwave energy heating source; means for delivering the foodstuff past said source; a magnetron connected to deliver microwave energy to said heating source; and power supply means connected for supplying operating power to said magnetron at a constant voltage, whereby the heating power from said source remains substantially constant as the load impedance presented by the foodstuff varies.

2. The apparatus defined in claim 1 wherein said magnetron produces a continuous microwave output.

3. The apparatus as defined in claim 1 wherein said power supply means comprises: a high voltage transformer having a primary winding and a secondary winding; and an induction voltage regulator connected to said primary winding.

4. In apparatus for the microwave heating of a foodstuff of the type which presents, with respect to microwaves, an electrical impedance whosemagnitude and phase angle vary by a substantial amount as the foodstuff is heated, which apparatus includes a microwave energy heating source and means for' delivering the foodstuff past such source, the improvement comprising: a magnetron connected to deliver microwave energy to said heating source; and power supply means connected for supplying operating power to said magnetron at a constant voltage, whereby the heating power from said source remains substantially constant as the load impedance presented by the foodstuff varies.

5. A method for the microwave heating of a foodstuff of a type which presents, with respect to microwaves, an electrical impedance whose magnitude and phase angle vary by a substantial amount as the foodstuff is heated, said method comprising: causing the foodstuff to flow past a microwave energy source so that the foodstuff constitutes part of the electrical load of the source; delivering microwave heating energy from the source to the foodstuff; generating the microwave heating energy in a magnetron forming part of the energy source; and supplying the magnetron with operating power at a constant voltage for maintaining the output power from the source substantially constant in response to variations in the load impedance presented by the foodstuff.

References Cited UNITED STATES PATENTS 2,648,772 8/1953 Dawson et al. 219-1055 X 2,847,637 8/1958 Grib 328-267 X 3,302,060 1/1967 Blok et al. 219-1055 X 3,321,314 5/1967 Jeppson 219-1055 X 3,263,052 7/1966 Jeppson et al. 219-1055 3,392,309 7/1968 Hickman 328-270 X OTHER REFERENCES McGraw-Hill Encyclopedia of Science & Technology (New York, 1960), vol. 14, pp. 365 and 366.

JOSEPH V. TRUHE, Primary Examiner L. H. BENDER, Assistant Examiner U.S. Cl. X.R. 331-186 

